146 lines
9.3 KiB
HTML
146 lines
9.3 KiB
HTML
<!DOCTYPE html>
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<html lang="zh" dir="ltr" class="client-nojs" xmlns="http://www.w3.org/1999/xhtml" xml:lang="zh">
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<head><meta http-equiv="x-ua-compatible" content="ie=edge">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
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<title>rand</title>
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<link rel="stylesheet" href="ext.css" />
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<link rel="stylesheet" href="site_modules.css" />
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-c_numeric_random_rand skin-cppreference2 action-view cpp-navbar">
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<div id="cpp-content-base">
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<div id="content"><a id="top"></a>
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<h1 id="firstHeading" class="firstHeading">rand</h1>
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<div id="bodyContent">
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<div id="contentSub"><span class="subpages">< <a href="c.html">c</a>‎ | <a href="c-numeric.html">numeric</a>‎ | <a href="c-numeric-random.html">random</a></span></div>
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<div id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
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<table class="t-dcl-begin">
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<div>定义于头文件 <code><stdlib.h></code></div>
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</td>
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<td></td>
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<td></td>
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<div><span class="mw-geshi c source-c"><span class="kw4">int</span> rand<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></span></div>
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<td class="t-dcl-nopad"></td>
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<td></td>
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<td></td>
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<p>返回 <span class="t-c"><span class="mw-geshi c source-c">​<span class="nu0">0</span>​</span></span> 与 <span class="t-lc"><a href="c-numeric-random-RAND_MAX.html">RAND_MAX</a></span> 间的随机整数值（包含 0 与 <code>RAND_MAX</code> ）。</p>
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<p><span class="t-lc"><a href="c-numeric-random-srand.html">srand()</a></span> 播种 <code>rand()</code> 所用的伪随机数生成器。若在任何对 <code>srand()</code> 的调用前使用 <code>rand()</code> ，则 <code>rand()</code> 表现如同它以 <code>srand(1)</code> 播种。每次以 <code>srand()</code> 播种 <code>rand()</code> 时，它必须产生相同的值数列。</p>
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<p>不保证 <code>rand()</code> 为线程安全。</p>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<p>（无）</p>
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<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
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<p><span class="t-c"><span class="mw-geshi c source-c">​<span class="nu0">0</span>​</span></span> 与 <span class="t-lc"><a href="c-numeric-random-RAND_MAX.html">RAND_MAX</a></span> 间包含边界的随机整数值。</p>
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<h3><span class="mw-headline" id=".E6.B3.A8.E6.84.8F">注意</span></h3>
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<p>无对产生的随机数质量的保证。过去，某些 <code>rand()</code> 的实现在随机性、分布和产生的数列周期中有严重缺陷（在一个广为人知的例子中，最低位在调用间简单地于 <code>1</code> 和 <code>0</code> 间改变）。不推荐将 <code>rand()</code> 用于严肃的随机数生成需求，如加密。</p>
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<p>POSIX 要求 <code>rand</code> 所用的伪随机数生成器的周期至少为 <span class="texhtml" style="white-space: nowrap;">2<span class="t-su">32<br /></span></span> 。</p>
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<p>POSIX 提供 rand 的线程安全版本，名为 rand_r ，它由于 <a rel="nofollow" class="external text" href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/drand48.html">drand48</a> 函数族而过时。</p>
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<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
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<div class="t-example">
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<div class="t-example-live-link"></div>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="c source-c">
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<pre class="de1"><span class="co2">#include <stdio.h></span>
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<span class="co2">#include <stdlib.h></span>
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<span class="co2">#include <time.h></span>
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<span class="kw4">int</span> main<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span>
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<span class="br0">{</span>
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<a href="c-numeric-random-srand.html"><span class="kw740">srand</span></a><span class="br0">(</span><a href="c-chrono-time.html"><span class="kw513">time</span></a><span class="br0">(</span><a href="c-types-NULL.html"><span class="kw103">NULL</span></a><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 以当前时间为随机生成器的种子</span>
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<span class="kw4">int</span> random_variable <span class="sy1">=</span> rand<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="c-io-fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">"Random value on [0,%d]: %d<span class="es1">\n</span>"</span>, <a href="c-numeric-random-RAND_MAX.html"><span class="kw741">RAND_MAX</span></a>, random_variable<span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 扔 6 面色子 20 次</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> n <span class="sy3">!</span><span class="sy1">=</span> <span class="nu0">20</span><span class="sy4">;</span> <span class="sy2">++</span>n<span class="br0">)</span> <span class="br0">{</span>
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<span class="kw4">int</span> x <span class="sy1">=</span> <span class="nu0">7</span><span class="sy4">;</span>
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<span class="kw1">while</span><span class="br0">(</span>x <span class="sy1">></span> <span class="nu0">6</span><span class="br0">)</span>
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x <span class="sy1">=</span> <span class="nu0">1</span> <span class="sy2">+</span> rand<span class="br0">(</span><span class="br0">)</span> <span class="sy2">/</span> <span class="br0">(</span><span class="br0">(</span><a href="c-numeric-random-RAND_MAX.html"><span class="kw741">RAND_MAX</span></a> <span class="sy2">+</span> 1u<span class="br0">)</span> <span class="sy2">/</span> <span class="nu0">6</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 注意： 1 + rand() % 6 有偏差！</span>
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<a href="c-io-fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">"%d "</span>, x<span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="br0">}</span></pre></div>
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</div>
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<p>可能的输出：</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="text source-text">
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<pre class="de1">Random value on [0,2147483647]: 448749574
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3 1 3 1 4 2 2 1 3 6 4 4 3 1 6 2 3 2 6 1</pre></div>
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</div>
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</div>
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<h3><span class="mw-headline" id=".E5.BC.95.E7.94.A8">引用</span></h3>
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<div class="t-ref-std-11">
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<ul>
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<li>C11 标准（ISO/IEC 9899:2011）：</li>
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</ul>
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<dl>
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<dd>
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<ul>
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<li>7.22.2.1 The rand function (p: 346)</li>
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</ul>
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</dd>
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</dl>
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<div class="t-ref-std-c99">
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<ul>
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<li>C99 标准（ISO/IEC 9899:1999）：</li>
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</ul>
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<dl>
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<dd>
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<ul>
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<li>7.20.2.1 The rand function (p: 312)</li>
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</ul>
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</dd>
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</dl>
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<div class="t-ref-std-c89">
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<ul>
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<li>C89/C90 标准（ISO/IEC 9899:1990）：</li>
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</ul>
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<dl>
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<dd>
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<ul>
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<li>4.10.2.1 The rand function</li>
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</ul>
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</dd>
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</dl>
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</div>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
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<table class="t-dsc-begin">
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="c-numeric-random-srand.html"><span class="t-lines"><span>srand</span></span></a></div>
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</div>
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</td>
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<td>播种伪随机数生成器<br />
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<span class="t-mark">(函数)</span></td>
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</tr>
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<tr class="t-dsc">
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<div class="t-dsc-member-div">
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<div><a href="c-numeric-random-RAND_MAX.html"><span class="t-lines"><span>RAND_MAX</span></span></a></div>
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</div>
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</td>
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<td><span class="t-c"><span class="mw-geshi c source-c">rand<span class="br0">(</span><span class="br0">)</span></span></span> 生成的最大可能值<br />
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<span class="t-mark">(宏常量)</span></td>
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</tr>
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</table>
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</div>
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</div>
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</div>
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<div class="visualClear"></div>
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</div>
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</div>
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</div>
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</body>
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</html>
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